Conventionally, a rubber plate for use in flexographic printing has been produced by etching a metallic plate to produce an original plate, preparing a matrix plate from, e.g., a plastic by using the original plate, and pouring a rubber into the matrix plate, followed by pressing. However, this method has drawbacks in that numerous steps are needed, thus requiring too great a cost and too much time. In addition, this method has also drawbacks in that the produced rubber plate itself is low in thickness precision and, hence, it is necessary to raise the thickness precision by shaving the back side of the plate before use.
In order to solve this problem, there has recently been proposed a method of producing a flexographic printing plate from a photosensitive resin composition. According to this method, not only can the step for the above-mentioned shaving of the back side of the plate be eliminated, but also it is possible to obtain a printing plate capable of printing a pattern which is finer, as compared to a pattern which can be printed by the conventional rubber plate.
As these photosensitive elastomer compositions, there are known, for example, those disclosed in Japanese Patent Application Laid-Open Specification No. 47-37521 (corresponding to British Patent No. 1366769), Japanese Patent Application Laid-Open Specification No. 51-106501 (corresponding to U.S. Pat. No. 4045231), Japanese Patent Application Laid-Open Specification No. 52-64301, Japanese Patent Application Laid-Open Specification No. 53-127004 (corresponding to U.S. Pat. No. 4197130), Japanese Patent Application Laid-Open Specification No. 54-110287 (corresponding to U.S. Pat. No. 4177074), Japanese Patent Application Laid-Open Specification No. 55-48744 (corresponding to U.S. Pat. No. 4266005) and Japanese Patent Application Laid-Open Specification No. 58-62640 (corresponding to U.S. Pat. No. 4431723).
Of these, a photosensitive elastomer composition in which a thermoplastic block copolymer is used as a binder (such as those disclosed in Japanese Patent Application Laid-Open Specification Nos. 47-37521 and 53-127004), has very excellent cold-flow resistance in an uncured state and has very excellent rubbery elasticity in a cured state and, hence, such a solid photosensitive elastomer composition has been mainly in practical use for producing a flexographic printing plate. Especially, a photosensitive elastomer composition, such as those disclosed in Japanese Patent Application Laid-Open Specification No. 53-127004 in which fumarate or maleate is used as an ethylenically unsaturated compound, exhibits excellent thermal stability at the time of heat molding for the production of a sheet therefrom and during the storage at high temperatures. Also, a printing plate obtained by using the above-mentioned composition has properties desired for a flexographic printing plate, such as good flexibility, so that the printing plate can be fittedly mounted onto the cylinder of a printing machine. In the above conventional photosensitive elastomer compositions, there is used a block copolymer in which the content of a monovinyl substituted aromatic hydrocarbon polymer block used is about 20 to 40% by weight, but the vinyl content of a diene compound polymer block used is generally low, namely, 15% or less.
However, a photosensitive elastomer composition comprising the conventional block copolymer as mentioned above, has problems mentioned below. First, these conventional photosensitive elastomer compositions are low in sensitivity and therefore, the time required for the insolubilization of compositions by photocuring, so that the compositions can be insoluble in a developer, is too long. In some cases, imagewise exposure requires as much as 20 to 30 minutes or more, thus causing the total time necessary for plate production to be long. In this instance, if the exposure time is reduced, a problem arises that the insolubilization of the photosensitive elastomer composition becomes insufficient, rendering it difficult to form relief portions corresponding to small dots and lines. On the other hand, if the intensity of light is increased, a problem arises that relief portions corresponding to small dots and lines become broad and that reverse lines are not formed.
Second, the conventional photosensitive elastomer compositions have a problem in that, unless the specific type of an ethylenically unsaturated compound to be used is selected, the photocured products are unsatisfactory in various mechanical properties, so that chipping is likely to occur especially in the relief portions corresponding to dots and lines during the development which is usually conducted with a solvent by means of a brush. In this case, there is a dilemma such that if a compound capable of preventing chipping from occurring at the time of development is selected as an ethylenically unsaturated compound, the obtained plate is poor in flexibility. Especially when fumarate or maleate is used, which, as mentioned above, is known to markedly improve the flexibility of a photocured plate and the thermal stability of a photosensitive composition before light exposure, the problem of the chipping of the relief becomes more serious.
On the other hand, in recent years, with an increasing demand for flexographic printing plates, the printing of finer images has been required in flexographic printing. For this reason, there is an increasing demand for forming smaller dots and characters. Accordingly, it has become very important to solve the problem that relief portions corresponding to such small dots and characters are damaged due to the occurrence of chipping during the operation of washing-out with an organic solvent by means of a brush after the light exposure. Further, a demand has been increased for the reduction of the plate production time, especially for the reduction of exposure time.
In these situations, the present inventors have made extensive and intensive studies with a view toward developing a photosensitive elastomer composition for producing a flexographic printing plate, which is not only capable of surely forming small dots and lines while reducing the plate production time, but also does not suffer from the occurrence of chipping in a photocured plate at the time of development, so that it can be especially useful for the production of a flexographic printing plate. As a result, the present inventors have found that when, with respect to a photosensitive elastomer composition comprising a thermoplastic block copolymer, an ethylenically unsaturated compound component and a photopolymerization initiator, there is used as the thermoplastic block copolymer a specific amount of a thermoplastic block copolymer comprising at least one monovinyl substituted aromatic hydrocarbon polymer block and at least one conjugated diene polymer block,
the block copolymer having a monovinyl substituted aromatic hydrocarbon content (A) of 10 to 35% by weight and PA1 the conjugated diene having a vinyl content (V) of 20 to 50% by weight, PA1 the block copolymer having a monovinyl substituted aromatic hydrocarbon content (A) of 10 to 35% by weight and PA1 the conjugated diene having a vinyl content (V) of 20 to 50%, PA1 wherein the (A) and the (V) satisfy the following formula: EQU 40.ltoreq.(A)+(V).ltoreq.70; PA1 the block copolymer having a monovinyl substituted aromatic hydrocarbon content (A) of 10 to 35% by weight and PA1 the conjugated diene having a vinyl content (V) of 20 to 50%, PA1 wherein the (A) and the (V) satisfy the following formula: EQU 40.ltoreq.(A)+(V).ltoreq.70,
wherein the (A) and the (V) satisfy the following formula: EQU 40.ltoreq.(A)+(V).ltoreq.70,
the photosensitivity of the composition is dramatically improved, so that the object of the development can be attained. The present invention has been completed, based on this finding.